Electrical induction apparatus construction



April 22, 1969 D. M. BARTOS ET L 3,440,587

ELECTRICAL INDUCTION APPARATUS CONSTRUCTION Filed April 11, 1967 Fly. 2

Fly. 1

IN VE N TORS ATTORE t United States Patent U.S. (ll. 336-206 7 Claims ABSTRACT OF THE DISCLOSURE Electrical induction apparatus, such as a transformer, having an insul-ative spacer between adjacent pairs of winding layers, the insulating spacer comprising a sheet of electrical insulating material having a flexible coating of insulating material on the surface of the sheet. The interstices between turns in each layer and between the layers of turns and said spacers are filled with an insulative impregnant having a substantially greater rigidity than the rigidity of the flexible coating. The flexible coating on the insulative spacer absorbs stresses caused by thermal cycling of the unit and prevents cracking of the relatively rigid impregnant which may be a cheaper material.

Background of the invention The present invention relates to electrical induction apparatus such as transformers, and is particularly related to the electrical insulation provided in such units.

Most power distribution transformers today are of the so-called liquid types; that is, they must be filled with oil to provide adequate electrical insulation. While oil-type transformers are certainly adequate for most applications certain inherent disadvantages are found in such units. First of all, oil deteriorates with time and use, and must periodically be checked for its dielectric strength. Oilfilled transformers must necessarily be relatively large, thus creating problems in pole mounting hardware and in underground distribution equipment, which is becoming increasely popular. The oil-filled transformer must necessarily be mounted in only one position due to the effect of gravity on the oil. Additionally, there is a potential fire hazard in the use of most transformer oils since heat is necessarily generated in these units.

Oil-filled transformers, in general, are constructed with kraft paper barriers between the high and low voltage windings. Since the units are designed to operate at relatively low temperatures and since the entire unit is immersed in oil, the paper insulattion is entirely satisfactory if one is to accept the premise that an oil-filled unit is optimum.

In relatively low power equipment, there are in use today many types of so-called dry-type transformers in which all insulating materials, while the unit is being used, are in a solid rather than liquid state. Such units are used, for example, in electronic equipment. It has heretofore been proposed that dry-type transformers can be used in power distribution systems.

The direct adaptation of small transformer design to high power units has been found to be impossible. First of all, the complete impregnation of a large transformer, even with low viscosity curable liquid insulating material, has ofen been found to leave voids in the insulation in large units even when impregnation is carried out in vacuum. If a void in the impregnant exists in the barrier between the high and low voltage windings, the insulation will break down at that point. Several designs have been proposed which provide large spacing units between transformer windings, thus placing sufficient air gap between Patented Apr. 22, 1969 windings to eliminate any problems of breakdown. Such units, however, lose the advantage of space economy which could be realized in a fully impregnated unit.

Most presently used electrically insulating impregnants cure into the relatively rigid materials. This is true, for example, of the epoxy resins, among others. While these materials have relatively high dielectric strength while they are intact, they are easily cracked or torn by physical stress. In relatively large induction apparatus, such as distribution transformers, variations in temperature during operation cause large thermal expansions and contractions which, in turn, generate physical stresses in any electrically insulating material. Such stresses cause cracks or tears and any void in the insulation, of course, is subect to the disadvantages hereinbefore related.

Summary of the invention specifically, in accordance with the present invention, an

electrically insulative spacer sheet of kraft paper, for example, is coated with a flexible insulation material such as a silicone rubber or gel; and the unit is interleaved between adjacent layers of windings of the induction coil. The interstices between the turns in each layer of the winding and between the layers of turns in the spacers are filled with an insulative impregnant having substantially greater rigidity than the flexible coating on the spacer.

The flexible coating thus acts as a stress absorbing member for changes in dimension in the unit and allows only minor amounts of stress to be absorbed by the impregnant.

Additionally, if desired, the flexible coating may contain embedded thereon a quantity of particulate insulating material which acts as a spacer to allow the free flow of uncured impregnant during construction of the device. Since the granular material is also embedded in the flexible material, there is no substantial increase in transmission of stresses to the impregnant.

The induction equipment produced in accordance with the present invention can be made extremely compact and light weight. Additionally, all of the problems inherent in oil-filled units are eliminated or substantially reduced. The necessity for maintenance is minimized. Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description when read in conjunction with the accompanying drawings.

Brief description of the drawing FIGURE 1 is a view in perspective of an electrical transformer made in accordance with the present invention;

FIGURE 2 is a cross-sectional view of the transformer of FIGURE 1 taken along the line 2-2 of FIGURE 1; and

FIGURE 3 is a fragmentary cross-sectional view of the transformer of FIGURES 1 and 2 taken along the line 33 of FIGURE 2.

Description of the preferred embodiments Referring now to the drawings wherein like reference characters represent like or corresponding parts throughout the figures thereof, there is shown in FIGURE 1 a transformer having an iron core 11. Placed around the iron core 11 are the transformer windings, shown generally as 12, which have pairs of leads 13 and 14 projecting therefrom. While the present invention will be described with respect to the simple two-winding transformer shown in the drawings, it will be realized that the same principles described herein are applicable to various other types of transformer designs as well as other types of electrical induction equipment.

Referring now to FIGURES 2 and 3, it will be seen that the windings 12 comprise a winding form 16 positioned in close proximity to the core 11, and around which is placed a low voltage winding 17, the insulating spacer 1'8, and a high voltage winding 19. As may be seen more clearly from FIGURE 3, the high voltage winding consists of a first layer of turns 21 in direct contact with the winding form 16. Continuing radially outward, a layer of sheet insulation material 22, of paper, for example, having a coating 23 of flexible electrically insulating material such as, for example, silicone rubber, and having embedded the-rein a plurality of particles 24, is placed in contact with the winding layer 21.

The particles 24 are designed primarily to act as spacers to allow free flow of impregnant into the unit during fabrication and may be eliminated if so desired. Suitable materials include grains of sand, glass beads, or particles of electrically insulating resinous materials suchas epoxy, for example. Surrounding the spacer member is a second layer 26 of low voltage turns which, in turn, is surrounded by a barrier layer comprising multiple turns of a spacer combination identical to that interposed between the winding layers 21 and 26. Formed on the exterior surface of the barrier layer 18 are the high voltage winding layers 27, 28 and 29, again interleaved with the insulative spacer combination of sheet, flexible coating, and granular particle. All interstices between turns in each layer and between the layers of turns and the spacer-s are filled with an insulative impregnant having in accordance with the invention substantially greater rigidity than the flexible coating on the insulative spacers. Suitable materials for the impregnants include, for example, epoxy resins and silicone resins, although any electrically insulating ma ter-ial which may be flowed into the unit in a liquid state and then solidified can be used.

In operation, as stresses due to variations in thermal expansion coefficient cause stresses in the insulation, the flexible coatings on the insulative spacers absorb the major portion of such stresses and prevent cracking or tearing of the relatively rigid impregnant. This type of configuration can therefore use a dielectric gel as the flexible coating and a relatively low tear strength encapsulant, as well as the elas-tomeric coating-resinous impregnant combination described hereinabove.

Obviously, many modifications and variations of the present invention will become obvious to those skilled in the art in view of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise thanas specifically described.

That which is claimed is:

1. In electrical induction apparatus of the type wherein multiple layers of turns of an electrically conductive wire lie in substantially concentric relationship with a substantially concentric web-shaped electrically insulative spacer member between each pair of adjacent layers, the improvement which comprises:

a flexible coating upon said insulative spacer, said coating constituting means for absorbing stresses caused by variations in thermal expansion; and

an insulative impregnant filling substantially all interstices between turns in each layer and between said layers of turns and the flexible coating on said spacers, said insulative impregnant being of substantially greater rigidity than said flexible coating.

2. The improvement defined in claim 1 wherein said flexible coating is a synthetic elastomer.

3. The improvement defined in claim 2 and further including a plurality of rigid spacer granules of insulative material embedded in said flexible coating.

4. The improvement as defined in claim 3 wherein said insulative impregnant is a synthetic resin.

5. The improvement defined in claim 1 and further including a plurality of rig-id spacer granules of insulative material embedded in said flexible coating.

6. The improvement defined in claim 5 wherein said flexible coating is a silicone elastomer, said rigid spacer granules are silica, and said insulative impregnant is an epoxy resin.

7. The improvement as defined in claim 1 wherein said flexible coating is a dielectric gel.

References Cited UNITED STATES PATENTS 961,805 6/1910 Scott 336-205 1,495,823 4/ 1924 Underhill 326-206 2,683,767 7/ 1954 Cunningham. 2,686,904 8/1954 Feder 336-206 X 2,743,308 4/1956 Bardsley. 2,914,600 ll/1959 Smith 336-96 2,951,277 9/1960 Youngs 161-209 2,993,949 7/1961 Moebius 161-209 3,210,701 10/1965 Fastner. 3,339,162 10/1967 Burnsteel 336-206 FOREIGN PATENTS 652,037 9/ 1957 Canada. 970,037 9/-l964 Great Britaain.

DARRELL L. CLAY, Primary Examiner.

E. A. GOLDBERG, Assistant Examiner.

U.S. Cl. X.R. 

